Label Allocation

In the JUNOS software, label values are allocated per router. The display output shows only the label (for example, 01024). Labels for multicast packets are independent of those for unicast packets. Currently, the JUNOS software does not support multicast labels.

Labels are assigned by downstream routers relative to the flow of packets. A router receiving labeled packets (the next-hop router) is responsible for assigning incoming labels. A received packet containing a label that is unrecognized (unassigned) is dropped. For unrecognized labels, the router does not attempt to unwrap the label to analyze the network layer header, nor does it generate an Internet Control Message Protocol (ICMP) destination unreachable message.

A packet can carry a number of labels, organized as a last-in, first-out stack. This is referred to as a label stack. At a particular router, the decision about how to forward a labeled packet is based exclusively on the label at the top of the stack.

Figure 1 shows the encoding of a single label. The encoding appears after data link layer headers, but before any network layer header.

Figure 1: Label Encoding

Figure 2 illustrates the purpose of the class-of-service bits (also known as the EXP or experimental bits). Bits 20 and 21 specify the queue number. Bit 22 is the packet loss priority (PLP) bit used to specify the random early detection (RED) drop profile. For more information about class of service and the class-of-service bits, see Configuring Class of Service for MPLS.

Figure 2: Class-of-Service Bits

Operations on Labels

The router supports the following label operations:

• Push—Add a new label to the top of the packet. For IPv4 packets, the new label is the first label. The TTL and S bits are derived from the IP packet header. The MPLS CoS is derived from the queue number. If the push operation is performed on an existing MPLS packet, you will have a packet with two or more labels. This is called label stacking. The top label must have its S bit set to 0, and might derive CoS and time to live (TTL) from lower levels. The new top label in a label stack always initializes its TTL to 255, regardless of the TTL value of lower labels.
• Pop—Remove the label from the beginning of the packet. Once the label is removed, the TTL is copied from the label into the IP packet header, and the underlying IP packet is forwarded as a native IP packet. In the case of multiple labels in a packet (label stacking), removal of the top label yields another MPLS packet. The new top label might derive CoS and TTL from a previous top label. The popped TTL value from the previous top label is not written back to the new top label.
• Swap—Replace the label at the top of the label stack with a new label. The S and CoS bits are copied from the previous label, and the TTL value is copied and decremented (unless the no-decrement-ttl or no-propagate-ttl statement is configured). A transit router supports a label stack of any depth.
• Multiple Push—Add multiple labels (up to three) on top of existing packets. This operation is equivalent to pushing multiple times.
• Swap and Push—Replace the existing top of the label stack with a new label, and then push another new label on top.

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